A sex-linked multiple allelic series at the P locus controls the onset of gonadotrop activity in the teleosts, Xiphophorus maculatus and X. pygmaeus. Indirectly, this gene controls the age or size at sexual maturity and adult size in males. The onset of sexual maturity of males can be accurately measured from the metamorphosis of the anal fin into a gonopodium. Experiments are designed to determine the number of alleles within natural populations and the phenotypes of the various allelic combinations. Special attention will be paid to certain P factors, pigment genes and sex chromosomes to determine whether linkage disequilibria exist. Pituitaries will be transplanted from genetically early donors into hypophysectomized late hosts to find out whether the age at which the gonadotrops differentiate is determined within the gland (genotype or donor specific) or elsewhere (host specific). The transplants will be placed close to the floor of the diencephalon ("in situ") because the gonadotrops of ectopic grafts are known to become nonfunctional. The use of isogenic strains and their hybrids circumvents the problem of histoincompatibility. In X. pygameus there is a relationship between growth rate during the first few weeks after birth and the size at sexual maturity. The somatotropic area of the pituitary gland of various genotypes will be examined for differential activity.